[Gretl-users] Scaled Frequency in Periodogram

Greetings, I am playing with sunspot data on a Sunday night, making periodograms in various software packages (R, SAS, ITSM, and Gretl). The Gretl periodogram is quite nice, with the period on the upper x axis and the "scaled frequency" on the lower x axis. None of the other packages present scaled frequency as a default (I don't know if they offer it at all), and I have been unable to nail down exactly what it is. Can anyone offer a simple explanation of scaled frequency as it is used in Gretl, or even better, a specific reference? Thanks. Tom